The present invention relates generally to multi-stage, field-coupled high frequency radio frequency (RF) filters and, more particularly, to multi-stage, field-coupled filters wherein the coupling window effective area is controlled as a function of frequency to provide a flat filter response over the filter tuning range.
Multi-section, varactor-tuned RF bandpass filters utilizing electromagnetic coupling between the filter stages are well-known in the art. However, for prior art field-coupled filters, a single physical adjustment of the coupling factor allows relatively small, typically, ten percent, frequency tune ranges. Further, inter-stage coupling at high frequencies, greater than 500 MegaHertz (MHz), for example, is difficult to control, and an adjustable but fixed coupling performs poorly over wide frequency range such as a 2:1 frequency span.
Tapped inductor and loop-coupled filters have very position-sensitive elements with respect to filter parameters such as insertion loss. Construction of physically smaller, lower-impedance loops reduces this sensitivity, but at the cost of substantial increases in frequency-dependent insertion loss. Electromagnetic field coupling between the filter stages is achieved by opening a physical "window" through the shield isolating the filter stages from one another thus allowing propagation of the filter inductor electromagnetic field. Electrical models of field-coupled filters are extremely complex; however, final physical designs can typically be easily reproduced with repeatable results.
Field coupling, similar to loop coupling, affects filter response shape, filter Q and insertion loss. Adjustment of the coupling window position, with respect to the inductor or resonator, will modify the filter response shape. Capacitive field coupling results from utilizing the electromagnetic field produced near the high impedance end of the resonant circuit while inductive coupling is associated with the low impedance end. A balance between the capacitive and inductive fields yields a relatively symmetrical filter frequency response. Further, adjustment of the coupling window area changes the amount of coupling and therefore changes the insertion loss and, to a lesser degree, the filter Q.